1. Technical Field
The present invention relates to jet pump and reactor and, in particular, to a jet pump and a reactor suitable for applying to a boiling water reactor.
2. Background Art
A conventional boiling water reactor (BWR) has a jet pump in a reactor pressure vessel (hereinafter referred to as an RPV) to which a recirculation pipe is connected. The jet pump has a nozzle, a bell mouth, a throat, and a diffuser. Cooling water in a downcomer, where the jet pump is disposed, formed in the RPV is pressurized by operation of a recirculation pump, pumped through the recirculation pipe as a driving flow, and ejected from the nozzle into the throat. The nozzle increases the speed of the driving flow. The cooling water around the nozzle in the downcomer is sucked into the bell mouth as a suction flow due to the working of the ejected driving flow, passes the throat, and flows into the diffuser. The cooling water discharged from the diffuser is supplied to a core through a lower plenum in the RPV (see, for example, U.S. Pat. No. 3,625,820, Japanese Patent Laid-open No. Sho 59(1984)-188100, Japanese Patent Laid-open No. Hei 7(1995)-119700, and Japanese Patent Laid-open No. 2007-285165).
Jet pumps disclosed in Japanese Patent Laid-open No. Sho 59(1984)-188100, Japanese Patent Laid-open No. Hei 7(1995)-119700, and Japanese Patent Laid-open No. 2007-285165 each have a plurality of nozzles. When the total area of each ejection opening formed in the plurality of nozzles remains constant, an increase in the number of nozzles increases the contact area between driving flows and suction flows, and thus mixing of the driving flows and the suction flows is promoted. Consequently, a mixing loss is decreased, increasing efficiency of the jet pump.
A jet pump installed in a reactor has a nozzle connected to a raiser pipe that is installed in the RPV. In this jet pump, an elbow pipe, the nozzle, a bell mouth and a throat are unified into one body, which structure allows the elbow to the throat to be removed for inspection and maintenance. A connection portion between the throat and the diffuser has a joint structure in which a lower end portion of the throat is inserted into an upper end portion of the diffuser. This joint structure is a slip joint. The slip joint, where the throat and the diffuser are connected, has a structure which allows the upper end portion of the diffuser and the lower end portion of the throat to slide up and down, so that no stress is generated due to the difference between the thermal expansions of the raiser pipe and the diffuser. For this reason, a gap is formed between an inner surface of the diffuser's upper end portion and an outer surface of the throat's lower end portion. Part of the cooling water that flows into the diffuser from the throat leaks out to the downcomer through the gap. This leakage flow prevents a foreign object from being caught in the gap or deposited on the surfaces. However, when the flow rate of the leakage flow exceeds a limit, the jet pump may start to vibrate. Thus, in order to suppress the vibration of the jet pump, the leakage flow from the gap in the slip joint should be limited below the limit.
Although it is not a jet pump, Japanese Examined Utility Model Application Publication No. Sho 52(1977)-5301 discloses a fluid sealing joint used for pipes for introducing high-temperature and high-pressure gas (or steam). In this fluid sealing joint, a tubular inlet-side joint portion is inserted into a tubular outlet-side joint portion; and an end portion of the inlet joint portion has a narrowing portion whose flow passage cross-sectional area decreases and an expanding portion whose cross-sectional area increases toward the end. A communication hole is formed in the place where the narrowing portion and the expanding portion are connected, the flow passage cross-sectional area of which the place is the smallest in the inlet-side joint portion. This communication hole communicates with an annular space portion formed between the inlet-side joint portion and the outlet-side joint portion. Static pressure inside is reduced at the seam between the narrowing portion and the expanding portion so that a fluid in the annular space portion is sucked inside the narrowing portion through the communication hole. This effectively prevents a fluid from leaking out of the fluid sealing joint through a gap between the inlet-side joint portion and the outlet-side joint portion.
Japanese Patent Laid-open No. Sho 59(1984)-159489 discloses a jet pump for suppressing vibration. In this jet pump, a lower end portion of a throat, which is inserted into an upper end portion of a diffuser, has a flow passage cross-sectional area that diminishes toward the end.
Other than that, for the purpose of reducing the amount of cooling water leaking from a slip joint of a jet pump, a way of forming a labyrinth seal on an outer surface of a lower end portion of a throat in the slip joint is known (see, for example, Japanese Examined Patent Application Publication No. Sho 59(1984)-48360).
A jet pump illustrated in FIG. 3 of Japanese Patent Laid-open No. 2001-90700 has a venturi tube and a nozzle that ejects a driving flow into the venturi tube a driving flow. This nozzle has an inner cylinder and an outer cylinder that surrounds the inner cylinder. A driving flow passage formed between the inner cylinder and the outer cylinder is an annular passage whose cross-sectional area gradually decreases towards the discharge side of the driving flow. The driving flow supplied to the driving flow passage is ejected from one end (a discharge opening) of the driving flow passage into the venturi tube. Cleaning water around the nozzle is sucked into the venturi tube due to the driving flow ejected from the nozzle. To be more specific, this cleaning water flows into the venturi tube through each of a first cooling water suction passage formed between the nozzle and the venturi tube and a second cooling water suction passage formed inside the inner cylinder. From the nozzle, the annular driving flow is ejected. Cross sections of the annular driving flow are similar to continuous rings.
Japanese Patent Laid-open No. 2008-82752 discloses a jet pump applicable to a BWR. This jet pump has a ring header for supplying a driving flow surrounding a suction flow suction passage formed in the center of the jet pump; and a nozzle portion installed to a lower end of the ring header, surrounding the suction flow suction passage, having a plurality of ejection openings in an annular arrangement, where the ejection openings eject a driving flow fed to the ring header.